Radio receivers and transmitters designed for use in selective call radio communication systems, such as paging systems, are often designed to have resonant frequency circuits, such as voltage controlled oscillators (VCOs) and tunable filters, which have controllable resonant frequency characteristics. A common design for VCO's includes one voltage variable tuning element having a variable reactance, used in a resonant, or tank, circuit having at least one fixed resonant element having a fixed reactance value, wherein a control voltage is impressed across the voltage variable tuning element to set a reactance value of the voltage variable tuning element. The resonant circuit is coupled to an amplifier and an oscillation voltage is generated which is impressed across each of the reactive elements. A commonly used fixed resonant element is an inductor. A commonly used voltage variable element is a varactor diode. A varactor diode has a voltage controllable capacitance which is variable over a capacitance range of approximately 2 to 1. The voltage controllable capacitance is controlled by a bias voltage, which is typically varied across an operational range of 0 to approximately 2 or 3 volts.
The resonant frequency tuning range of the VCO is determined, among other things, by the reactance values of the fixed elements and the range of the reactance of the variable tuning element. Because the reactance of the variable tuning element in a VCO is sensitive to voltage changes, VCO's typically have an inherent problem of generating undesirable side-band noise which is partially caused by a phenomenon known as self-modulation. This phenomenon of self-modulation is caused by the bias voltage being modulated by the oscillation voltage. The resulting modulated bias voltage induces the undesirable side-band noise in the oscillation voltage. The self-modulation effect can be reduced by reducing the sensitivity of the variable reactance element to changes of the bias voltage, but this results in a narrower frequency range of the VCO.
One approach to providing a desired tuning range while using a low sensitivity variable reactance element is to use the low sensitivity variable reactance element in combination with one or more fixed amounts of reactance, such as capacitors which are switched into or out of the resonant circuit. This, in effect, gives two or more tuning ranges which can be tuned by the one variable reactance element. These ranges can be overlapped to achieve the desired total tuning range. Approaches using PIN diodes, field effect transistor switches and additional varactors have been used, but they have one or more of the drawbacks of requiring additional current drain, additional control and ground connections, additional parts, and can also increase side-band noise.
Thus, what is needed is a simpler circuit for providing frequency networks having a wide tuning range with less side band noise and lower current drain in portable and mobile radios.